1. Field of the Invention
The present invention relates to an electromagnetic wave transforming device for transforming an electromagnetic wave represented by high energy radiant rays such as X-rays or γ-rays or radiant rays such as α-rays into electric signals.
2. Related Background Art
Up to now, the electromagnetic wave transforming device using X-rays among electromagnetic waves represented by high energy radiant rays such as X-rays or γ-rays, α-rays or β-rays has an imaging device portion for image input and a detecting device portion that detects the irradiation quantity of X-rays, separately, those portions being connected as independent devices in use.
In the above structure, the X-ray quantity in the imaging device is controlled on the basis of the X-ray irradiation quantity detected by the detecting device.
In this situation, in the case where a detecting element for detecting the X-ray quantity is located in front of the imaging device portion of the X-ray imaging device to detect the X-rays, there is a case in which an image obtained by the imaging device is adversely affected by the detecting element.
Subsequently, an example of a conventional X-ray automatic exposure control device will be described.
The automatic exposure control device (photo timer) in the X-ray photographing is used in most of indirect photographing and direct photographing and widely spread.
The automatic exposure control device operates so as to hold the X-ray quantity irradiated onto a film to a desired range by automatically controlling a photographing time. The X-rays that have passed through an object to be photographed is transformed into an electric signal, and when an integral value of the electric signals reaches a given value, the X-rays are blocked (or the irradiation of the X-rays stops) so as to obtain a desired film density.
The automatic exposure control devices are classified into various types in accordance with the X-ray detecting mechanism and a control mechanism such as a method of determining the X-ray exposure conditions.
An example of the structure of the automatic exposure control in a transmission fast photographing is shown in FIG. 11.
In the figure, the X-rays that have passed through an object to be photographed is converted and amplified into a visible light by I.I. (image intensifier) 901. A part of the output light of the I.I. 901 is guided to a photoelectron multiplexing tube 903 through a distributor 902 and then transformed into an electric signal. The electric signal is integrated by a capacitor 904. The integral value corresponds to the degree of exposure of the film and is compared with a reference value set by a film density setter 905 by a comparator 906, and when the integral value reaches the set value, an X-ray block signal is generated to stop the irradiation of the X-rays.
Subsequently, a conventional detector will be described.
As means for detecting the X-ray that has passed through the object to be photographed, there are an I.I. natural lighting type, a cassette front face fluorescent lighting type, a cassette rear face fluorescent lighting type, an ionization box type and a semiconductor detection type that is being now studied. Those types can be roughly classified into the front face natural lighting system and the rear face natural lighting system.
Both of the cassette front face fluorescent lighting type and the ionization box type are of the (cassette) front face natural lighting system in which a detector is disposed between a film and an object to be photographed.
The I.I. natural lighting type and the cassette rear face fluorescent lighting type are of the rear face natural lighting system in which the X-rays that have passed through the film is detected.
FIGS. 12A and 12B show an example of a fluorescent lighting type detector in which the thickness of the detector is about 2.5 mm which is significantly thinner than that of the ionization box (the thickness of about 10 mm). The detecting mechanism of the fluorescent lighting type is that the X-rays that have passed through the object to be photographed are applied to a fluorescent paper (phosphor) 911 to fluoresce the phosphor. Then, the fluorescence in a hollowed portion of a light block paper 912 is guided to a light guide plate 913, and reaches the photoelectron multiplexing tube 914 so as to be inputted to the photoelectron multiplexing tube 914 to conduct photoelectric conversion.
As described above, in the case where the X-ray amount detecting element is disposed in front of the X-ray imaging device to detect the X-rays, because the detecting element receives a part of image information before the electromagnetic waves such as the X-rays reach an imaging region, obtained image information may be adversely affected.
Also, the cassette front face natural lighting system is more influenced by the X-ray absorption and scattering rays due to the exposure quantity detection than the rear face natural lighting system, as a result of which the image quality may be deteriorated, or the exposure quantity may increase in order to supplement the absorption. As the thickness of the detector is thicker, a distance between the object to be photographed and the film becomes longer with the result that geometrical blur becomes large, to thereby deteriorate the image quality. This is because the X-ray tube focal point is displaced from the image receiving position of a film or a sensor which is an imaging section because of the thickness of the detector.
In addition, in case of the rear face natural lighting system, the X-ray absorption between the X-ray source that makes the film photosensitive and the detector is normally large, and there are many cases in which the tube voltage dependency characteristic is deteriorated. Also, when the rear face absorption is reduced in order to reduce the tube voltage dependency characteristic, the film is liable to be adversely affected by the backward scattering rays generated when the X-rays that have passed through the film are irradiated onto a backward member.